| Due to extensive application, lithium ion batteries have been researched widely in recent years. Comparing with anode materials, cathode materials due to lower specific dishcharge capacity and higher cost become the key factors to limit litium-ion batteries development. So far, layered materials containing colbat still occupy a pivotal position in cathode material markets. With the shortage of cobalt mineral resources, the price of cobalt mineral will rise and that will cause the cost of lithium-ion cathode materials to dramatically rise. Thus, adopting inexpensive and eco-friendly element iron as the alternative of colbat will reduce the cost of lithium-ion battery cathode materials significantly. In addition, manganese-rich layered materials can improve the discharge specific capacity and enhance the cycle capability under the high potential, and thus increasing the content of manganese in ternary materials has some significance.Based on the above object, the experiment uses facile sol-gel method to prepare manganese-rich Li-Fe-Ni-Mn-O materials. The p H of solution and the ratio of lithium salt and totally transition metal salts in the process of sol-gel method was optimized, and the optimal parameters are obtained: the p H of solution is 4, and the ratio of lithium salt and totally transition metal salts is 1.1:0.9.In order to improve the stability of material at high potential and thermal stability, Zn O, Cu O and Al2O3 coating were used to modifying the manganese-rich Li-Fe-NiMn-O material. The results show that above coating prepared by subsequent coat method do not change the crystal structure of the materials. Except that Zn O coating has the influence on the morphology, other metal oxide coatings barely effect the morphology. All metal oxide coatings positively contribute to the cycle stability in ambient and high temperature. Coated materials were tested by electrochemistry impedance spectroscopy under different potential to investigate the coating influence on the stability during cycles. The results evaluate that coatings restrain the increment of resisitance of medium-high semicircle and decomposition of electrolyte on the surface of cathode materials under high potential. Further, activation phenomenon is hardly observed in Zn O coating materials due to Zn O coating owning higher electron conduction, and this phenomenon can be observed in other metal oxide coating because of lower electron conduction. However, the activation phenomenon in other metal oxide coating materials will not influence the discharge specific capacity after the finish of activation phenomenon. Adopting a certain amount of metal oxide coating can enhance the discharge specific capacity under high current condition. Materials with 1mass% Zn O coating, 1mass% Cu O coating, 3mass% Cu O coating and 1mass% Al2O3 coating discharge better specific capacity of 71.7m Ah/g, 77.7m Ah/g, 70.3m Ah/g and 70.0m Ah/g, which are more than uncoated material.In order to suppress the impurity and improve the diffussion of lithium ion, aluminum substituting manganese, aluminum substituting iron and sodium substituting lithium in bare material were attempted. Among these, aluminum substituting manganese materials have finite contribution to suppres impurity, and compared with manganese, aluminum cannot provide enough structure stability. In this way, although aluminum substituting manganese materials show higher initial discharge specific capacity, the cycle capability is weakened. Aluminum substituting iron can suppress impurity in bare materials very well, but the higher discharge specific capacity could only found in more than half substitution materials. When the substitution is 0.15, the material shows the best discharge specific capacity and rate performance. Synergy study of Nyquist and Bode plots perform that 0.15 mol aluminum substitution can increase electron conduction and hamper the formation of solid electrolyte interface layer. As for sodium substituting lithium materials, when a little amount of sodium substitute lithium, the structure of material is enhanced, but with the amount increase, the impurity begings appearing in the materials. Sodium substitution materials perform the larger discharge specific capacity especially under higher charge/discharge current condition. Galvanostatic intermittent titration technique is used to calculate the diffusion of lithium ion, and the results inform that sodium substitution materials have large lithium ion diffusion than bare materials, which is the reason that sodium substitution materials own the ability of higher discharge specific capacity under large charge/discharge current. |
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